There are numerous circumstances where health, safety, and/or product quality can be affected by gas-borne or air-borne dusts or micro-organisms. Accordingly there exists a requirement for a method and apparatus enabling rapid and accurate monitoring of dusts or micro-organisms, preferably with selectivity in respect of the range of size or type of air-borne particulate that is monitored, and it is an object of the invention to provide such a method and apparatus.